This invention relates to thyristor bridges. More specifically, this invention relates to a thyristor protection circuit that allows for a reduced size and improved placement of reactors necessary to protect the thyristors.
Thyristor bridges are solid state electrical devices that allow precise control of voltage and current. They are often controlled by logic units that interpret a broad range of signals and allow the thyristors to conduct over very precise intervals. Because of their controllability, they are often used in a broad range of applications such as analog to digital or digital to analog converters, power conversion devices, and frequency modulating devices.
Thyristor bridges are often used to control electric motors or start electric generators. In order to accomplish these tasks, control of both the frequency and magnitude of the magnetic field is preferable. Therefore, electric generator starters often include a thyristor bridge circuit for power conversion. A thyristor bridge may be employed to convert standard line voltage (such as generally available 60 Hz or 50 Hz alternating current electricity) into a direct current supply. This direct current supply may then be manipulated by a second thyristor bridge into alternating current having the desired frequency, magnitude, and phase angle.
The thyristors offer signal controllability, but they are often expensive and need to be protected from rapid increases in voltage across them and currents through them. FIG. 2 demonstrates one method to protect the thyristors 201-206 in a bridge configuration having large leg reactors 207-212. These large reactors often must be externally mounted around bus bars in series with a thyristor stack. The thyristor protection circuit may also have snubber circuits 213-218 placed in parallel with the thyristors 201-206. The thyristors 201-206 may be controlled by respective logic control units 219-224. FIG. 3 demonstrates a snubber circuit comprising a resistor 301 in series with a capacitor 302. The configuration here has some deficiencies such as high cost and mounting difficulties. Other problems with thyristor protection circuits also exist.
The present invention overcomes the problems discussed above, and provides additional advantages, by employing a new circuit to protect thyristors in a bridge configuration. The invention modifies an existing bridge circuit by adding line reactors to the respective AC inputs of the diametric cells of a thyristor bridge. Among other benefits, this allows a reduction in the leg reactors that may be required in a traditional design. Part of the invention is a new snubber circuit added in parallel with the thyristors which, among other considerations, prevents excessive switching dv/dt that would otherwise occur due to the addition of the AC line reactors. In summary, the combination of the leg reactors, line reactors, and snubber circuits operate to protect the thyristors from rapid rates of anode current and voltage, unreasonably high peak voltage, and other limiting specifications.
One embodiment of the invention comprises a thyristor bridge rectifier device having diametric cells connected between DC bus lines, the diametric cells comprising two thyristors, line and leg reactors in series with the thyristors, and two snubber circuits in parallel with respective thyristors. In another embodiment of the invention, the aforementioned thyristor bridge rectifier is adapted to and employed in a load commutated inverter static starter system. Another embodiment of the invention is a method of protecting a thyristor in a bridge configuration. The method comprises the steps of operatively associating each thyristor in the bridge configuration with a leg-line-reactor protection mechanism.